LM3464MHX National Semiconductor Corporation, LM3464MHX Datasheet - Page 12

• Manufacturer Part Number: LM3464MHX
• Description: Lm3464 Led Driver With Dynamic Headroom Control And Thermal Control Interfaces
• Manufacturer: National Semiconductor Corporation

www.national.com
As the system enters steady state, the rail voltage V
creases and finally settles to an optimal level that maintains
the maximum power efficiency of the entire system. The volt-
age level of V
lowing this equation:
In the equation, V
V
carry the highest forward voltage among the four LED stings.
V
the headroom voltage for the LM3464 driver stage and equals
to the minimum V
FETs under steady state. The VDHC pin is internally biased
to 0.9V which also set the default voltage headroom to 0.9V.
In applications that the output of the AC/DC converter con-
tains more than 0.9V peak-to-peak ripple voltage, the voltage
headroom can be increased by applying external bias to the
VDHC pin.
DEFINING VOLTAGE HEADROOM
The voltage headroom is the rail voltage margin that reserve
for precision linear current regulation under steady state. Un-
der steady state, the voltage headroom is always minimized
by the LM3464 to reduce power losses on the MOSFETs till
one of the drain voltage (V
voltage on VDHC pin (0.9V typical).
With external bias, the voltage of the VDHC pin can be ad-
justed up or down to adapt to different types of primary power
supply. Figure 4 shows a simple resistor based biasing circuit
that derives biasing voltage from the output of the internal
voltage regulator, the VCC pin.
With the additional resistors, the VDHC pin voltage is ad-
justable in between 0.8V and 2V. The values of R
should be at least 10 times lower than the values of the inter-
nal resistor divider of the VDHC pin. However, it is recom-
mended not to set the voltage headroom too low because the
ripple voltage of the primary power supply output may and
cause visible flicker due to insufficient voltage headroom.
Thus the voltage headroom follows this equation:
FIGURE 4. Adjusting Voltage Headroom with Resistors
f(highest)
VDHC
is the voltage at the VDHC pin. This voltage decides
is the total forward voltage of the LED string which
RAIL
V
RAIL
DRx
under steady state can be calculated fol-
RAIL
among the drain voltage of the MOS-
is the rail voltage in steady state and
= V
f(highest)
DRx
) of the MOSFETs equals the
+ V
VDHC
30115013
A
RAIL
and R
de-
B
12
where 0.8V < V
SETTING LED CURRENT
The LED current regulating mechanism of the LM3464 driver
stage contains four individual LED current regulators. Every
LED current regulator is composed of an external MOSFET
(Q
amplifier inside the LM3464 that monitors the feedback volt-
age from the current sensing resistor. The integrated amplifier
compares the voltage across current sensing resistors
(R
trols the gate voltage of the MOSFETs (Q
linear current regulations. Figure 5 shows the simplified circuit
of the linear LED current regulators.
The driving currents of the LED strings are defined by the
values of R
the value of R
Since R
resistors should be resistors with no more than 1% tolerance
and adequate rated power to the desired LED current.
ISNS1
1
-Q
4
), a current sensing resistor (R
-R
ISNS1
FIGURE 5. Linear LED Current Regulator
ISNS4
ISNS1
FIGURE 6. LED Current vs R
to R
ISNSx
) to a 200mV typical reference voltage and con-
VDHC
to R
ISNS4
is related by the following equation:
< 2V
ISNS4
define and carry LED currents, these
individually. The LED current and
ISNS1
ISNSx
-R
1
-Q
ISNS4
4
30115011
) to realize
) and an
30115029